Semiconductor sensors that sense the presence or movement of a magnetic field are commonly referred to as “Hall effect” or “Hall” sensors. Hall sensors have many applications. For example, Hall effect sensors are commonly used in brushless direct current (DC) motor applications for two purposes—to commutate the motor and to provide motor position feedback. Hall sensors are solid state devices and accordingly have no moving parts to wear out. Use of Hall sensors may consequently minimize maintenance requirements and improve longevity for various applications, such as for brushless DC motors.
A digital Hall sensor is a Hall sensor with certain signal conditioning circuitry that produces an output that is one of two states that represent digital values of 1 or 0. The signal conditioning components and/or circuitry may include a low noise, high input impedance differential amplifier, and a threshold or trigger stage. The trigger stage triggers one of the digital output values when the output of the Hall element crosses a certain threshold value. The digital output state of the Hall sensor will not change unless the input value has crossed the particular set threshold value. The trigger stage is commonly a Schmitt trigger or Schmitt inverter.
The digital Hall sensor signal conditioning circuitry may also include an output transistor, e.g., a current sinking NPN transistor. The digital Hall sensor output may be defined or characterized by the electrical characteristics of the output transistor, which may include the type of the output transistor, e.g., NPN, the maximum current, the breakdown voltage, and the switching time. The current sinking output transistor may be configured in an open collector configuration. In some applications, the output of the output transistor may be floating and a pull-up resistor may be used to help establish a solid quiescent voltage level.
FIG. 1 shows a representative brushless DC motor 100 including a rotor 101 with multiple rotor magnets, e.g., permanent magnets 102a–102d having alternating north and south pole orientations, mounted on an accessory shaft 104 having a non-driven end 104a and a driven end 104b. For an 8-pole application, four corresponding rotor magnet pairs (omitted for clarity) would normally be mounted around the hidden side of the accessory shaft 104. Stator windings 106 are shown surrounding the rotor 101 and rotor magnets 102a–102d. The stator windings 106 may be mounted on the inside of a motor housing (not shown).
To rotate the rotor 101, the stator windings 106 are energized in a sequence. The rotor position relative to the stator determines the order in which the windings will be energized so that the rotor moves in a desired direction, e.g., clockwise (CW) or counter clockwise (CCW). The rotor position is sensed using one or more Hall sensors, for example, Halls sensors 110a–110c, as shown. For the commutation sequence, a first winding is typically energized to positive power, i.e., with current entering into the winding, and a second winding is negative, i.e., with current exiting the winding, while a third winding is in a non-energized condition. For subsequent steps in the commutation sequence, the windings are energized accordingly to cause the rotor to spin within the stator.
The Hall sensors 110a–110c are mounted on a stationary part of the motor 100, e.g., on the inner periphery of the stator. The Hall sensors 110a–110c may be mounted to detect the movement of the rotor magnets 102a–102d or, in certain applications, to detect the movement of optional Hall sensor magnets 112a–112d (with four omitted for clarity) that are scaled-down versions of the rotor magnets. Hall sensor magnets 112a–112d are typically mounted to the accessory shaft 104 away from the driven end 104b of the shaft. The Hall sensors 110a–110c are commonly mounted on a printed circuit board and may be fixed to an enclosure cap on the non-driven end of the rotor 101. A controller 114 may be used to control the energizing sequence of the windings 106 and to keep the motor 100 running at a desired speed and in a desired direction.
FIG. 2 shows the combined output, or digital values, produced by three digital Hall sensors 202a–202c. The combined output is indicated by three Hall lines 204a–204c and is shown for rotation angles over a single rotation of a typical prior art 8-pole brushless DC motor application, e.g., motor 100 in FIG. 1. The Hall sensors 202a–202c may, for example, correspond to Hall sensors 110a–110c in FIG. 1, and may be placed 30 degrees apart relative to rotor 101. A Hall state transition occurs in the combined output when the digital state of one of the Hall lines, e.g., 204a, changes in value, for example from a 0 to a 1 as shown in FIG. 2 at 30 degrees of rotation.
During operation of the motor, e.g., motor 100, the rotating permanent magnet rotor moves across the front of each digital Hall sensor 202a–202c and causes the sensor to change state. Each digital Hall sensor 202a–202c operates as a particular pole, e.g., a south pole, of a rotor or Hall sensor magnet passes that particular sensor, triggering a digital state, e.g., either 1 or 0. When an opposite pole, e.g., a north pole, passes the same digital Hall sensor, that sensor may release, triggering the complementary digital state, i.e., 0 or 1. For the Hall output sequence shown in FIG. 2, 24 state counts are produced in a 360-degree motor rotation, i.e., six distinct hall states times four cycles.
For the prior art output 204a–204c shown in FIG. 2, noise may cause the output 204a–204c to indicate an erroneous or false Hall state transition, i.e., a transition to a false Hall state that does not correspond to actual movement of the rotor and magnets. When digital Hall sensors are used for motor position feedback and commutation, a noise-induced Hall state transition may indicate a false position of the rotor, which can lead to faulty commutation and degradation in motor performance.